The number of birth defects among twins and triplets increased nearly two-fold in 14 European countries between 1984 and 2007, according to a new study.

Researchers, who had information on more than 5.4 million births occurring over the 24-year period, found that congenital defects rose from about 6 in every 10,000 multiple births to about 11 in every 10,000 multiples.

"The importance of knowing this is twofold. First, to make sure we have appropriate services available for mothers and babies. The second is to understand the relationship," said Helen Dolk, the study's senior author.

Dolk, from the Center for Maternal Fetal and Infant Research at the University of Ulster in Northern Ireland, said she and her fellow researchers knew multiple births were on the rise and that those babies were at an increased risk for birth defects.

For the new study, the researchers, who published their findings in BJOG: An International Journal of Obstetrics and Gynecology, looked at the trends in births in 14 European countries between 1984 and 2007.

Of the 5.4 million births, the researchers found the number of multiple births increased by about 50 per cent over that time. Ultimately, 3 per cent of the births were multiple births.

Of the 148,359 major birth defects within those births, about 4 per cent occurred in babies who were multiples.

Over the 24-year period, though, the number of birth defects among multiples about doubled, with a peak between the years 2000 and 2003 of some 12 birth defects per 10,000 multiple births.

The largest increase was in birth defects not caused by chromosomal abnormalities, such as physical deformations - which increased from about six per 10,000 multiple births between 1984 and 1987 to about 11 per 10,000 births at the end of the study.

Compared to singleton births, the researchers found the risk of birth defects was about 27 per cent higher for multiples.

According to the researchers, some of the rise in birth defects may be attributed to the increased use of in vitro fertilisation (IVF), which is known to come with an increased risk of anomalies.

The researchers write that people going through the IVF process might consider only implanting one embryo into a womb. While that may not cut down on the risk of a birth defect, it may affect the pregnancy outcome and put less of a strain on parents and resources.

"The fundamental message remains: the risks are low. Most babies are born healthy from multiple births or (IVF), but there are a number of reasons why single embryo transfers is a better option than multiple embryo transfer," Dolk said.

In general, she added, people should talk with their doctors before conceiving about reducing the risk of delivering a baby with a birth defect.

"The discussion with the doctor should be before pregnancy and that doesn't happen very often," she said.